The recent evolution of handheld communication devices, from simple mobile phones to more complex multifunctional smartphones, has seen a significant increase in size of certain components in the devices. In particular, the size of the liquid crystal display (LCD) has grown so that the user is able to visualize more information, larger pictures and videos, and also so that the display may be operated more easily as a touchscreen. The battery has also grown in size in order to provide sufficient power to drive the larger LCD, faster microprocessors, and other components.
Simultaneously, the size of other components in the devices has had to be reduced in order to accommodate the more consumer focused components such as the larger LCD and battery. In particular, there has been constant progress in miniaturizing the antennas used by the terminal to communicate with radio base stations (cellular), satellites (GPS), hot-spots (WiFi) and other terminals (Bluetooth®).
The miniaturization of cellular antennas poses significant technical challenges, especially since operating demands on the antenna are often increased. Novel, miniaturized antennas are expected to maintain or even increase the operating frequency ranges in order to be able to cover ever more communication protocols, in different frequency bands. The introduction of the Long Term Evolution (LTE) standard places further demands on the requirements of cellular antennas. In some countries, LTE requires the use of the 698 MHz to 798 MHz frequency band. The corresponding wavelength for this frequency range is large and would typically necessitate larger antennas, in stark contrast with the requirement for miniaturizing the antennas. Moreover, the LTE standard requires a multiple-in multiple-out (MIMO) antenna system, which requires the use of multiple antennas working in the same frequency band with a similar level of performance. Therefore, the LTE standard requires fitting at least two cellular antennas in an LTE-capable handheld terminal, leaving even less space for each single antenna.
FIG. 1 shows a schematic representation of a typical modern cellular handheld device 100. The terminal shows two cellular antenna elements 102, 101. The first antenna element 102 is placed at the bottom end of the printed circuit board (PCB) 104 containing the electronic components. This arrangement maximizes the space available for the LCD display 106 and the battery 105. The second antenna element 101 is often placed along one of the long edges of the device 100. However, because limited space is available (due to the large display 106 and battery 105) the second antenna 101 normally has inferior performance compared to the first antenna 102, and is typically used only for diversity reception, not being suitable for full MIMO applications. The performance of the second antenna 101 is also hindered by its position as the second antenna 101 is likely to be covered by the user's hand during normal use of the device 100. The antenna free area 108 at the top of the device 100 is usually kept free from antenna elements because positioning a second antenna in this area may generate high electromagnetic fields in the user's head, and therefore cause the device 100 to fail the regulatory compliance tests (e.g. Specific Absorption Rate (SAR) and Hearing Aid Compatibility (HAC) tests).
A convenient antenna arrangement is known from US 2011/0199267, where two L-shaped antenna are positioned in adjacent corners of a handheld telecommunications device. This arrangement reduces the influence the user has on the antenna characteristics by minimizing the electromagnetic currents on the PCB of the device, where the PCB typically functions as the groundplane for the antennas. However, this arrangement suffers from a severe limitation if one wants to use a similar concept at low frequency (<1 GHz). In fact, it is well known from D. Manteuffel, A. Bahr, D. Heberling, and I. Wolff, “Design considerations for integrated mobile phone antennas,” in Proc. 11th Int. Conf. Antennas Propagat., 2001, pp. 252-256, that in order to achieve sufficient bandwidth at low frequency using a compact antenna in a handheld device, the antenna must be capable of exciting electromagnetic currents on the large conductive parts of terminal (PCB, LCD, or chassis), so the antennas can function as efficient, low-Q radiators. In US 2011/0199267, the antenna does not appear to excite sufficiently large electromagnetic currents on the conductive parts of the terminal to be able to operate as an efficient, wide frequency band antenna at low frequencies. An electromagnetic simulation performed using the antenna parameters disclosed in US 2011/0199267 shows that its lowest resonant frequency is around 1730 MHz, as shown in FIG. 2. Scaling the antenna of US 2011/0199267 so that it is able to operate below 1 GHz would result in an impractically large antenna with narrowband behaviour.